In a cellular communication system, there is need for distribution of a lot of different information between different nodes, stationary as well as mobile. One type of information that has become more and more interesting is the location of a mobile unit. Up to now, the lateral position, i.e. the position in a horizontal plane, e.g. the latitude and longitude of the position has encountered most interest, but also the altitude is of increasing importance.
One typical application, where altitude determination becomes important is in the position determination of a mobile unit. By using travelling time measurements for signals travelling to a mobile unit from a number of base stations, a number of distances are thus determined. Each distance corresponds to a circle around each base station and the mobile position can in principle be determined as the common intersection of all circles. For a perfectly flat surface, distance measurements from at least three base stations are needed in order to secure a unique intersection. If the altitude of the mobile differs considerably from the altitude of at least one of the base stations, also the third dimension has to be considered. The distances then generate spheres, and to fully and uniquely determine the position of the mobile station, at least four measurements are needed. However, since the altitudes of the base stations typically are relatively similar, the accuracy in altitude determination becomes poor. Addition of the altitude information in any other way may improve and simplify the position determination of a mobile station.
Different approaches for distributing and using altitude information may be possible, and one possibility would e.g. be to concentrate on using the present lateral position and pre-established altitude maps to obtain the present altitude by a table lookup. A stationary node may easily digitally store such map data related to the area associated with its own cell. However, mobile units, moving from one cell to another, can not generally store altitude maps for all possible cells. Also when a stationary node needs information about an altitude of a position outside its own cell, all information may not be stored within the same unit in practical cases. Altitude information thus typically has to be communicated between different nodes in a cellular communication system.
One possible solution is that if a mobile unit wants to determine its altitude, the mobile unit communicates with a stationary unit, sending e.g. its present lateral position coordinates. The stationary unit processes the coordinates and returns an altitude value. However, if there is a need for an intermittently updated altitude determination, a lot of signaling has to be performed, which requires a certain bandwidth and hence reduces the remaining capacity of the cellular communication system, in particular when many users are active. It may therefore be a better solution if the mobile unit by itself may be able to determine the altitude, with a minimum of repeated communication.
An approach to such a solution is to communicate data representing an altitude map from a stationary node to the mobile station, when the mobile station enters into a certain cell, e.g. together with a handover procedure.